The State of the Ca Isotope Proxy

At the Earth's surface, Ca is a critical element at a variety of scales. It is both a biological nutrient and water-soluble, and is a major constituent of the dominant mineral sink for carbon in the ocean. Additionally, the 5‰ range in the stable isotope ratios of Ca (⁴⁴Ca/⁴⁰Ca) suggests that Ca isotopes may be a promising tracer of the Ca cycle, specifically the oceanic budget over time. Despite ~15 years of concentrated effort on high-precision Ca isotope measurements, the utility of Ca isotopes as a proxy remains far from clear. A variety of basic questions have yet to be resolved, both in the marine and terrestrial realms. To provide perspective, the current work presents a data compilation of over 60 published Ca isotope studies. The compilation includes δ^44/40Ca_SRM-915a measurements of the modern Ca cycle, including rivers and groundwater, dust, soils and soil pore fluids, vegetation, rainwater, silicate minerals/rocks, and marine carbonates. The focus of this work is to quantify the leverage of inputs to change the isotopic composition of the ocean. One of the tenets of the weathering proxy is that there is little isotopic leverage to change seawater. If this assumption is valid, then significant variations in the isotopic composition of seawater can be explained <u>to some extent</u> by mass flux imbalances between Ca inputs and outputs, requiring the Ca cycle to be out of steady state for significant periods of time. Despite evidence that Ca fractionates in the modern system during continental cycling, the δ⁴⁴Ca range of riverine inputs to the ocean is very narrow (especially when compared to the spread in marine carbonates). Thus, there appears to be minimal isotopic leverage amongst inputs to shift the ocean δ⁴⁴Ca. In order to develop our understanding of the Ca isotope proxy, we identify two probable mechanisms for shifting ocean δ⁴⁴Ca and evaluate them using a series of simple box models. In the terrestrial realm, plants exhibit a wide range of δ⁴⁴Ca (~3.5‰). Because there is compelling evidence that Ca fluxes via biomass degradation are significant at the catchment scale, it is important to assess the conditions under which the continental biosphere can influence riverine, and hence seawater, δ⁴⁴Ca. Utilizing isotopic fractionation factors for biological uptake derived from the literature, we employ models to constrain the conditions under which vegetation is relevant to the Ca isotopic evolution of the ocean. In the marine realm, we evaluate the importance of the global fractionation factor for varying seawater δ⁴⁴Ca and suggest methods by which such variability can be distinguished from changes in the δ value of the weathering flux. Experimental data suggest that there is considerable leverage (<1‰) to change the global fractionation factor. Identifying such changes requires measuring two separate phases, one of which samples seawater and one of which represents the global output from seawater. We present simulations that characterize the effect of fractionation factor variability, and discuss how such variability may be produced. Ultimately, such an interpretation requires robust and well understood proxies, which should be the goal of future research.